The old render system gave each bucket to a core.  And if you had a very compute intensive area you would see a core or two working and working while the rest finished and just sat there doing nothing.  The new system puts all the cores to work on the same area so that intensive area will get more processing power to complete faster and you don't have 14 of 16 cores doing nothing.

Poser 5, 6, 7, 8, Poser Pro 9 (2012), 10 (2014), 11, 12, 13